Screw for an extruding device and a device including said screw

ABSTRACT

Screw for a plate-type extruder. The screw comprises at least one screwthread which starts, at the root of the screwthread, in the form of an arcuate spade portion and has in a portion of the screwthread adjacent the spade portion a cross-sectional shape having a concave crest. The root of the screwthread has a conventional cross-sectional shape having a crest parallel to the axis of the screw.

United States Patent Wagner, deceased Sept. 5, 1972 [54] SCREW FOR ANEXTRUDING DEVICE AND A DEVICE INCLUDING SAID SCREW Inventor: ClaudeJulien Rene Wagner,

deceased, late of Pont-A-Mousson,

France by Paulette Marie Josephine Wagner, administratrix Assignee:Sidel,

Filed: Dec. 10, 1970 Appl. No.: 96,732

Le Havre, France [56] References Cited UNITED STATES PATENTS 3,280,23910/ I966 Ninneman ..425/376 X 3,577,589 5/1971 Serrano ..425/37s x2,994,105 8/1961 Seal et al. ..425/376 x Primary Examiner-J. SpencerOverholser Assistant ExaminerDavid S. Safran Attorney-John Lezdey andMarks & Clerk 57 ABSTRACT Screw for a plate-type extruder. The screwcomprises at least one screwthread which starts, at the root of thescrewthread, in the form of an arcuate spade portion and has in aportion of the screwthread adjacent the spade portion a cross-sectionalshape having a concave crest. The root of the screwthread has aconventional cross-sectional shape having a crest parallel to the axisof the screw.

' I 9 7 &///// 5 Claims, 8 Drawing Figures PATENTEDSEP 5 1912 SHEET 2[IF 2 SCREW FOR AN EXTRUDING DEVICE AND A DEVICE INCLUDING SAID SCREWThe present invention relates to plate-type extruders for extrudingplastics materials and elastomers in general and to screws for conveyingand compressing gelled materials issuing from the extruder so as toeject them under pressure through anorifice of an extruding or shapingdie. More particularly, the invention relates to a screw mounted in theaxial extension of a platetype extruder, said screw being provided withscrewthreads defining helical grooves which communicate with a spacebetween the plates or airgap for receiving the gelled material in theaxial zone of the extruder and conveying it to the outlet die at apressure sufficient for production on an industrial scale.

It is known that a plate-type extruder, or extruder employing a sheareffect, converts a plastics material, such as rigid polyvinyl chloridein the form of powder or grains, into a gelled material which issues byway of an axial outlet orifice at a pressure which is insufficient toovercome the resistance to flow of a narrow outlet orifice andconsequently at a rate of flow which is insufficient for production onan industrial scale. There is above all a danger of decomposition of thegelled material owing to excessive heating consequent to its excessivelyslow passage through the extruder.

This is why attempts have been made to increase the flow by means of ashort screw of conventional type (archimedes screw) which is rotated athigh speed and results in a pressure of the gelled material which issufficient to ensure expulsion at industrial rates of flow through theoutlet die. However, such a screw does not give the desired results.

The Applicant has found that the poor results obtained appear to be dueto a poor flow of the material in the zone of passage from theplate-type extruder to the conveyor screw, so that the screw, which isdesigned for high rates of flow, is insufficiently supplied withmaterial.

The object of the invention is to provide a screw of the aforementionedshort type adapted to be mounted in the axial extension of a plate-typeextruder so as to convey and compress the gelled material issuing fromthe extruder, this screw being so improved as to remedy theaforementioned drawback and overcome the resistance to flow of anextrusion die having a high pressure drop across its orifice, inparticular a narrow die.

The invention provides a screw adapted to be mounted inside a sleeve atthe outlet end of an extruder, said screw comprising at least one threadwhich starts, at the root of the screw, in the form of an arcuate spadeportion and has adjacent said spade portion in a first portion of itslength a section having a concave crest, the rest of the threadfollowing on said first portion having a section which has aconventional rectilinear crest parallel to the axis of the screw.

Owing to this spade-shaped portion of the thread of the screw at itsroot, the screw is supplied with material in a correct and sufficientmanner to ensure an industrial rate of flow through an extrusion diehaving a high pressure drop across its orifice.

Consequently, the rate of flow of the gelled material is markedlyincreased and becomes sufficient for production on an industrial scaleand, moreover, the flow can be maintained constant and regular and thepassage of the material through the extruder is at a rate which is highenough to avoid over heating so that the material produced is sound.

Another object of the invention is to provide an extruding device whichcomprises a plate-type extruder whose airgap communicates through aconvergent connection zone having a convex section with an axialcylindrical sleeve followed by an extrusion die, and a screw rotatablein said sleeve, wherein said screw comprises at least one thread whichstarts at the root of the screw, near the rotating plate of theextruder, in the form'of an arcuate spade portion and has adjacent saidspade portion in a first portion of its length a section having aconcave crest which matches the convex shape of said convergentconnection zone, the rest of the thread following on said first portionhaving a section which has a conventional rectilinear crest parallel tothe axis of the screw.

Further features and advantages of the invention will be apparent fromthe ensuing description with reference to the accompanying drawings.

In the drawings:

FIG. 1 is adiagrarnmatic sectional view of a platetype extruderaccording to the invention;

FIG. 2 is partial perspective view, on an enlarged scale, of the screwof said extruder;

FIG. 3 is a partial sectional view corresponding to FIG. 1 on anenlarged scale of the connection zone between the airgap of theplate-type extruder and the screw;

FIG. 4 is a sectional view of the screw taken along line 4-4 of FIG. 3;

FIG. 5 is a diagrammatic sectional view of a detail of the device shownin FIG. 3;

' FIG. 6 is a view similar to FIG. 3, the screw being in anotherposition relative to the sleeve, and

FIGS. 7 and 8 are views similar to FIG. 5 of two modifications of theinvention.

In the embodiment shown in FIGS. 1-6, the invention is applied to anextruder B having a cylindrical dish-shaped body 1 having a verticalaxis XX. Formed on the cylindrical wall of the body is a part for ahopper 2 supplying the material to be extruded. This hopper is arrangedin such manner that the material poured therein flows in a directionparallel to the axis XX and opens onto the face of the bottom of thebody 1 at a distance from the bottom. Rotatable in the cylindricalcavity of the body 1 in the direction of arrow f is a rotor 3 driven bya shaft 4 having an axis of rotation coinciding with the axis XX. Amotor (not shown) drives the shaft 4. The bottom of the body 1 and therotor 3 constitute the two plates of the extruder E.

The body 1 is axially extended by a cylindrical sleeve 5 which containsa screw V. The sleeve 5 has, internally and in its upper part, a zone 6constituting a convergent connection zone which has a convex section andconnects the bottom of the body or stator 1 to the cylindrical cavity inwhich the screw V is disposed. The cylindrical sleeve 5 terminates atits lower end in an extruding die 7 including for example a core 8.

The screw V has a shape suitable for conveying and compressing thematerial gelled by the extruder E. Therefore, it is much shorter thanextruding screws of which it has only the end portion for conveying andcompressing the material. This screw is mounted on the rotor 3, forexample bymeans of a centering plate 9 and a square portion 10, fortransmitting the drive. The screw V has threads 11 having asubstantially trapezoidal .cross section which matches or marries upwith the face of the cylindrical cavity of the sleeve 5. There are forexample two threads.

Each thread 11 at the root of the screw near the rotor 3 starts with acylindrical spade portion 12 whose width a (FIG. 6), measured in thedirection parallel to the axis X-X, is equal to that of the connectionzone between the airgap e (space between the plates 1 and 3) and thecavity of the sleeve 5. The spade portion 12 has an edge 13 which isnormal to the surface of the stator and of the rotor 3 and serves toremove or scoop the gelled material. Perpendicular to the edge 13, thespade portion 12 has a helical edge 14 which cn-' stitutes the start ofa thread 1 1. This helical edge 14 has a concave cross-sectional shapewhich matches that of the convex and convergent face '6 of the body 1and forms-progressively, in a first portion of the thread, the crest ofthe section of this thread. The first portion of the thread 11 having aconcave crest is progressively converted into a thread having atrapezoidal section with a rectilinear crest parallel to the axis X-X.The thread retains this section throughout the rest of its length. l

Each cylindrical spade portion 12 is extended by a helical groove whichextends completely round the screw (see FIGS. 2 and 6 in respect of thehelical edge 14).

The screw V terminates for example in a pointed portion 16 (FIG. 1) inthe zone of the extruding die 1 in the known manner.

It will be understood that the extruder E and the screw include theconventional heating and cooling means (not shown).

The device operates in the following manner The rotor 3 is rotated inthe direction of arrow f. The material to be extruded, for example rigidpolyvinyl chloride in the form of grains or powder, is supplied to theairgap E by the hopper 2. The plastics material is frictionally engagedbetween the plates 1 and 3, driven in the direction from the peripherytowards the center, softened by the heat and gelled when it arrives inthe connection zone 6 leading to the cavity of the sleeve 5. In thiszone the material is scooped by the edge 13 of each spade portion 12 andintroduced in the helical groove 15 extending from the other edge 13 ofthe screw V. The plastics material is thus trapped in the helicalgrooves 15 and is conveyed by the screw to the extruding die 7 fromwhich it issues in the form of a tubular blank or parison.

Note that owing to the spade portions 12, the width a of each spadeportion and the concave shape of the helical edge 14 of the spadeportion which constitutes the crest of the starts of the threads 11, theplastics material encountered by the spade portions 12 is rapidlyconveyed to the helical grooves 15 of the screw with no possibility offlowing back in the upstream direction, that is, towards the plates 1and 3. Indeed, by

- matching the convex and convergent face 6 of the sleeve 5 and owing toits cylindrical shape, each spade portion 12 partly closes the zone ofpassage from the airgap e to the helical groove 15 which precedes the Vedge in the direction of rotation of the screw and removes a largeamount of the material in the airgap.

' The screw V is consequently capable 'of supplying the plasticsmaterial at a rate of flow and at a pressure which are much higher thanthe'rates of flow and pressures of screws of conventional type, thepressure being capable of reaching 2-300 bars, notwithstanding thenarrow passage of the extruding die 7. The rate of flow and the pressureare also much higher than those of plate-type extruders which are notequipped with such a screw. Thus, as compared to a given plate-typeextruder, the rate of flow is ten times higher, it varying, for anavailable 15 HPv (French), between 60 and kg/hour, depending on the"section of the annular space between the screw and the sleeve 5 and thespeed of rotation of the rotor 3 and screw. By way of comparison, notubular blank can be obtained in a suitable manner with a screw ofconventional type mounted in the extension of a plate-type extruder Ewhich is devoid of the spade portions 12.

In. respect of more powerful and larger extruders equipped with thescrew according to the invention, the rates of flow may be much higher,of the order of several hundreds of kilograms/hour, with pressures asmuch as 200-300 bars or more.

The invention is applicable to the production of solid blanks or ringsand tubular blanks of different sections to be subsequently put into therequired shape.

According to a modification shown in FIG. 7, instead of having acylindrical shape, the spade portion 12a has a conical shape andterminates in an oblique edge 13a. This arrangement enables the plasticsmaterial to be scooped or taken further from the axis X-X in the airgap,between the rotor 3 and the stator 1 without excessively spreading thepart of the sleeve 5 which adjoins the body 1.

According to another modification shown in FIG. 8, instead of having anedge 13 normal to the plate of the stator l, a cylindrical spade portion12b can have a curved convex edge 13b so as I to scoop the gelledmaterial more efficiently.

It must be understood that the invention is also applicable to anextruder E having a horizontal axis X- X. It is also applicable to thecase where the screw B is rotated independently of the rotor 3 and canconsequently be driven at a speed which differs from the speed ofrotation of the rotor and even in the opposite direction.

Having now described my invention what I claim and desire to secure byLetters Patent is 1. An extruding device comprising an extruder having astator, a rotor plate rotatablerelative to said stator about an axis, anairgap between said plate and said stator, a cylindrical sleeve havingan axis extending along said axis of rotation and an end portionintegral with said stator, an extruding die combined with a portion ofsaid sleeve remote from said end portion, a convergent connection zonehaving a peripheral convex inner face putting said airgap incommunication with said sleeve, and a rotary screw disposed in andcoaxial with said sleeve and defining an annular space with said sleeve,said screw comprising at least one screwthread which starts, at the rootof said screwthread adjacent said plate, in the form of an arcuate spadeportion and has in a portion of the screwthread adjacent said spadecoaxial with said axis and'has an edge for scooping gelled material tobe extruded, said edge being parallel .to said axis.

4. An extruding device as claimed in claim 1,

wherein said spade portion has a part-conical shape and an edge forscooping gelled material to be extruded, said edge being obliquerelative to said axis.

5. An extruding device as claimed in claim 1, wherein said spade portionhas an edge in the form of a convex curve.

1. An extruding device comprising an extruder having a stator, a rotorplate rotatable relative to said stator about an axis, an airgap betweensaid plate and said stator, a cylindrical sleeve having an axisextending along said axis of rotation and an end portion integral withsaid stator, an extruding die combined with a portion of said sleeveremote from said end portion, a convergent connection zone having aperipheral convex inner face putting said airgap in communication withsaid sleeve, and a rotary screw disposed in and coaxial with said sleeveand defining an annular space with said sleeve, said screw comprising atleast one screwthread which starts, at the root of said screwthreadadjacent said plate, in the form of an arcuate spade portion and has ina portion of the screwthread adjacent said spade portion across-sectional shape having a concave crest which matches and engagessaid convex face of said convergent connection zone, the rest of thescrewthread following on said first portion having a cross-sectionalshape which has a conventional rectilinear crest parallel to the axis ofsaid sleeve.
 2. An extruding device as claimed in claim 1, wherein thewidth of the cylindrical spade portion of said screwthread is equal tothat of the connection zone between said airgap and said annular space.3. An extruding device as claimed in claim 1, wherein said spade portionhas a part-cylindrical shape coaxial with said axis and has an edge forscooping gelled material to be extruded, said edge being parallel tosaid axis.
 4. An extruding device as claimed in claim 1, wherein saidspade portion has a part-conical shape and an edge for scooping gelledmaterial to be extruded, said edge being oblique relative to said axis.5. An extruding device as claimed in claim 1, wherein said spade portionhas an edge in the form of a convex curve.